Tomorrow a certain time traveling doctor celebrates a 50th birthday. We're talking about Doctor Who. The British science fiction TV series predates "Sesame Street," even "Star Trek." And Saturday it celebrates its first 50 years with a 3D TV special to be simulcast in 75 countries. The BBC has also produced a documentary about the history of a show that has a fanatical following around the world. NPR TV critic Eric Deggans offers a simple reason for why the Doctor has maintained his appeal for so long.

ERIC DEGGANS, BYLINE: At the heart of England's greatest adventure series is the Doctor; maddening, eccentric, dashing, and oh-so-English.

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DEGGANS: He travels through space and time, fighting evildoers threatening the Earth. Still, if you believe the BBC's new made-for-TV movie about the creation of Doctor Who, the original star, William Hartnell, had to be sweet talked by a producer into the role.

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DEGGANS: Even the name of the show sounds like a fortuitous slip of the tongue. But there's really one reason why the Doctor can still charm TV audiences 50 years on: regeneration. When a time lord's body wears out, he can replace it with a completely new one. Producers pounced on this idea while scrambling to replace an ailing Hartnell, a moment recreated for the BBC's movie.

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DEGGANS: Regeneration turned an awkward change into a showcase moment, complete with flashing lights and dramatic sounds. But the actor who plays today's Doctor, Matt Smith, revealed its true showbiz value in a BBC documentary.

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DEGGANS: That's right. When an actor gets tired of the role, or fans get tired of him or the show needs a little spicing up, producers can just switch out the star. And regeneration also keeps "Doctor Who" on the pulse of pop culture. When James Bond was hip in the late '60s, dashing Jon Pertwee played the doctor as action hero.

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DEGGANS: And a few years later, when Monty Python's humor electrified British TV, curly-haired Tom Baker was a daffy Doctor with a penchant for jelly baby candies.

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DEGGANS: Today's Doctor, Matt Smith, is a bow tied nerd with the face of an underwear model, perfect for the age of Facebook. Some things never change. He's always British, always white and always a he. Later this year, another flash of light will anoint Peter Capaldi as the 12th Doctor. Nerds familiar with past episodes know there may only be 13 total.

But if Capaldi works out, they'll probably find a way to craft a new string of Doctors. After all, there's a grand British TV tradition to uphold.